Is there any one long exposure setting that works in all situations?
Question came in from Ron
There is no definite answer to this, but I typically start the long exposure 10 to 5 minutes before the end of dusk (check gaisma.com for your local dusk time) using f/11 or f/13 with a base shutter speed of 2 or 2.5 or 3 seconds (which is extended to 128, 160 and 192 seconds respectively with 6 stop ND [neutral density] filter attached). Let me unpack this a bit.
Shoot Within 10 to 5 Minutes Before the End of Dusk
Most of my blue hour cityscapes are shot within 10 to 5 minutes before the end of dusk. If shot too early (e.g. 20 minutes before the end of dusk), there are not many city lights available, and the sky is still too bright to do any sufficiently long exposure.
On the other hand, if shot too late (e.g. after the end of dusk), the beautiful bluish hue in the sky is gone and replaced with muddy-coloured dark sky, which I think is rather ugly (that’s why I never shoot after dusk).
Use Narrow Aperture (f-stop) like f/11 or f/13
I use a narrow aperture like f/11 or f/13 because (i) it captures greater depth of field (getting more of the scene in focus) and (ii) helps slow down shutter speed by letting in less light. For example, when having 1 second of shutter speed at f/8, switching to f/11 gives you 2 seconds of shutter speed, as f/11 lets in half as much light as f/8.
Use Base Shutter Speed of 2 to 3 Seconds
Once I set the aperture at f/11 or f/13 on Aperture Priority mode, I start a “waiting game”, i.e. just wait until shutter speed comes down to 2 to 3 seconds. As the light falls, shutter speed naturally gets longer towards dusk, and we get 2 to 3 seconds of shutter speed (at f/11 or f/13) around 10 minutes before the end of dusk here in Singapore.
Once this is achieved, I quickly switch to Manual mode, attach 6 stop ND filter and start long exposure for either 128 or 160 or 192 seconds (depending which base shutter speed is used, either 2 or 2.5 or 3 seconds respectively) by referring to Long Exposure Calculator app .
Both photos were shot almost at the same timing (about 11 minutes before the end of dusk at f/13, using a base shutter speed of 3 seconds), with no ND filter attached (left) and 6 stop ND filter attached (right), which extended the exposure to 195 seconds. As you can see clearly, the photo on the right got the water smoothed out much more and city lights were brighter, all thanks to the extended shutter speed (i.e. long exposure).
When to Start the Long Exposure Affects the End Result
Lastly, there is one particular thing that affects the end result, i.e. when to start the long exposure. This sounds like common sense, but photos shot at the deep end of dusk come out darker and more vibrant while those shot earlier somewhat lack vibrancy but look cleaner.
First photo: 18mm, f/13, 195 seconds (based shutter speed of 3 seconds, with 6 stop ND filter attached), ISO 100, shot 11 minutes before the end of dusk.
Second photo: 18mm, f/11, 268 seconds (based shutter speed of 4 seconds, with 6 stop ND filter attached), ISO 100, shot 4 minutes before the end of dusk.
As seen above, starting the exposure 11 and 4 minutes before the end of dusk made a significant difference in results. I personally prefer the first shot for its clean look (second shot is a little too vibrant for my liking). Whatever your preference is, you always have full control to choose when to start the exposure, so you can nail it!
Getting Dark Too Early? Open up the Aperture And/Or Shoot with Even Longer Exposure
N.B. At a place with fewer city lights, or when shooting the direction that is 180 degrees opposite where the sun set, the scene gets darker much faster, and base shutter speed of 2 to 3 seconds (at f/11 or f/13) can be achieved earlier than 10 minutes before the end of dusk. In such cases, open up the aperture (to e.g. f/8) and wait for the shutter speed to come down to 2 to 3 seconds again.
Besides, shoot with even longer exposure (e.g. base shutter speed of 4 and 5 seconds can be extended to 256 and 320 seconds respectively with 6 stop ND filter attached) or the combination of both (opening up the aperture AND using a longer shutter speed).